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1、附录1.英文原文AnOverviewofCDMACDMABasicsCDMAsystemsspreadaslowinformationbitratewithafastchipsequence,transmititovertheairandretrievetheoriginalinformation.HowtoactuallyspreadandretrievetheinformationisstandardizedindetailsinIS-95.Threemaintoolsareused:Walshcodes,64-chiport
2、hogonalsequencesAshortcode:215-1=32767chipslong,whichhasthepropertyofbeingorthogonaltoanynonzerooffsetofitself.Alongcode:242chipslong,usedtogenerateuniquesequences,whicharepseudo-orthogonaltoone-another.Thefollowingsectionsgointofurtherdetailsonwherethesetoolsareusedi
3、nvariousaspectsoftheIS-95airinterface:onforwardlinks,reverselinks,inaccessmode,andintrafficmode.WalshCodesForwardLinkWalshcodesareorthogonalcodes,IS-95usesthemtomultiplexseveralmobilecommunications(andcontrolchannels)ontheforwardlink.Intheforwardlink,eachmobileusesasp
4、ecificWalshcodesequence;allsequencesaremultiplexedtogetherinatotalcombinedsequence:(wheresirepresentstheinformationsymbol(+1or-1),giistheindividualchannelgain.)Thatsequenceismanipulatedfurtherandsentovertheair;onthereceiverside,thatsequenceisdecodedbysimplyintegrating
5、foreachchannel:forchannelk,theinformationbitisretrievedfromthesignoftheintegral:ReverseLinkInthereverselinkofIS-95,Walshcodesarenotusedinthatmannerbutsimplyencodebitsina64-aryencodingscheme:thatseach6-bitsequenceismappedtoa26=64bitWalshcode.InfurtherevolutionslikeIS-2
6、000andIS-856,thereverselinkmultiplexesseveralchannelssimilarlytotheforwardlink,andOrthogonalpropertiesoftheWalshcodesprovidethemultiplexingscheme:ShortCodeShortcodesarebitsequenceswithveryspecificautocorrelationfunctions.IS-95usesa215-1=32767chipshortcode.Forafirstexa
7、mpleletususeherea22-1=3bitlongshortcode:100.Permutated,thatshortcodehasthefollowingproperty:if+1iscountedforeverybitofthepermutatedsequencethatisidenticaltothebitoftheoriginalsequence,and-1iscountedwhenthatbitisdifferent,everypermutatedsequencetotals-1,whereastheorigi
8、nalsequenceobviouslytotalsitslength.Ifwenotetheabovecomputation,wehaveforashortcodesequence:Thisisaremarkablepropert
